Study of the Direct CO2 Carboxylation Reaction on Supported Metal Nanoparticles

catalysts

Article Study of the Direct CO2 Carboxylation Reaction on Supported Metal Nanoparticles

Fabien Drault 1 , Youssef Snoussi 1, Joëlle Thuriot-Roukos 1, Ivaldo Itabaiana, Jr. 1,2 ,Sébastien Paul 1 and Robert Wojcieszak 1,*

1 Univ. Lille, CNRS, Centrale Lille, Univ. Artois, UMR 8181-UCCS-Unité de Catalyse et Chimie du Solide, F-59000 Lille, France; [email protected] (F.D.); [email protected] (Y.S.); [email protected] (J.T.-R.); [email protected] (I.I.J.); [email protected] (S.P.) 2 Department of Biochemical Engineering, School of Chemistry, Federal University of Rio de Janeiro, Rio de Janeiro 21941-910, Brazil * Correspondence: [email protected]; Tel.: +33-(0)320-676-008

Abstract: 2,5-furandicarboxylic acid (2,5-FDCA) is a biomass derivate of high importance that is used as a building block in the synthesis of green polymers such as poly(ethylene furandicarboxylate) (PEF). PEF is presumed to be an ideal substitute for the predominant polymer in industry, the poly(ethylene terephthalate) (PET). Current routes for 2,5-FDCA synthesis require 5-hydroxymethylfurfural (HMF) as a reactant, which generates undesirable co-products due to the complicated oxidation step. There-

fore, direct CO2 carboxylation of furoic acid salts (FA, produced from furfural, derivate of inedible lignocellulosic biomass) to 2,5-FDCA is potentially a good alternative. Herein, we present the pri- mary results obtained on the carboxylation reaction of potassium 2-furoate (K2F) to synthesize

2,5-FDCA, using heterogeneous catalysts. An experimental setup was ﬁrstly validated, and then several operation conditions were optimized, using heterogeneous catalysts instead of the semi-

Citation: Drault, F.; Snoussi, Y.; heterogeneous counterparts (molten salts). Ag/SiO2 catalyst showed interesting results regarding Thuriot-Roukos, J.; Itabaiana, I., Jr.; the K2F conversion and space–time yield of 2,5-FDCA. Paul, S.; Wojcieszak, R. Study of the

Direct CO2 Carboxylation Reaction Keywords: carboxylation; metal nanoparticles; heterogeneous catalysis; FDCA; furoic acid on Supported Metal Nanoparticles. Catalysts 2021, 11, 326. https:// doi.org/10.3390/catal11030326 1. Introduction Academic Editor: Sophie Hermans Recently, the production of 2,5-furandicarboxylic acid (2,5-FDCA) from biomass has awakened interest [1–9]. 2,5-FDCA is one of the most important building blocks for the Received: 14 February 2021 production of polymers, such as the poly(ethylene furandicarboxylate) (PEF), which can Accepted: 2 March 2021 replace poly(ethylene terephthalate) (PET), derived from terephthalic acid (TA), a non- Published: 4 March 2021 sustainable molecule. Two main routes have been studied in the literature for the 2,5-FDCA synthesis from C6 or C5 compounds transformation. Moreover, 5-hydroxymethylfurfural Publisher’s Note: MDPI stays neutral (HMF) oxidation to 2,5-FDCA has been widely studied, and although it has shown to with regard to jurisdictional claims in published maps and institutional afﬁl- have the best catalytic results, some problems regarding 2,5-FDCA selectivity are found iations. due to the formation of unstable intermediate products [4–6,10–14]. Moreover, HMF is quite unstable and provokes serious problems during the oxidation process. In addition, HMF generally obtained from fructose need to be of very high purity. On the other hand, 2,5-FDCA synthesis from hemicellulose-derived chemicals is of great importance. Indeed, furfural could substitute HMF, as its industrial production from non-edible resources Copyright: © 2021 by the authors. is a mature process. Production of furoic acid synthesis from furfural oxidation, using Licensee MDPI, Basel, Switzerland. heterogeneous catalysts in a alkaline [15–17] or base free media [6,18], has been studied. This article is an open access article distributed under the terms and Then, the C–H carboxylation of furoic acid with CO2 can form 2,5-FDCA (Figure1). This conditions of the Creative Commons reaction has shown to be more selective than that from HMF [19–21]. Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Catalysts 2021, 11, 326. https://doi.org/10.3390/catal11030326 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, x FOR PEER REVIEW 2 of 11

CatalystsCatalysts 20212021,, 1111,, x 326 FOR PEER REVIEW 2 2of of 11 10

Figure 1. Reactional scheme of CO2 carboxylation of furoic acid to 2,5-furandicarboxylic acid (2,5- FDCA). FigureFigure 1. 1. ReactionalReactional scheme ofof COCO22carboxylation carboxylation of of furoic furoic acid acid to 2,5-furandicarboxylicto 2,5-furandicarboxylic acid (2,5-FDCA)acid (2,5- . FDCA).However, the main problem in the 2,5-FDCA synthesis from furoic acid is the car- However, the main problem in the 2,5-FDCA synthesis from furoic acid is the carboxy- boxylate group insertion on hydrocarbon C-H bonds [22,23]. As C1 feedstock, CO2 pre- lateHowever, group insertion the main on hydrocarbonproblem in the C-H 2,5-FDCA bonds [22 synthesis,23]. As C1from feedstock, furoic acid CO is2 presentsthe car- sentsthermodynamic thermodynamic and kineticand kinetic limitations limitations [24]. [24] Indeed,. Indeed, in the in esteriﬁcationthe esterification of aromatic of aromatic hy- 2 boxylatehydrocarbons group with insertion CO2, aon low hydrocarbon equilibrium C-H conversion bonds [22,23]. at every As temperature C1 feedstock, is COobtained pre- sentsdrocarbons thermodynamic with CO2 ,and a low kinetic equilibrium limitations conversion [24]. Indeed, at every in the temperature esterification is obtained of aromatic [24]. [24].Consequently, Consequently, several several solutions solutions have beenhave studiedbeen studied to perform to perform the direct the direct C–H carboxylation, C–H carbox- hydrocarbons with CO2, a low equilibrium conversion at every temperature is obtained ylation,by using by a using base asa base a reagent, as a reagent, as previously as previously developed developed by Kolbe by Kolbe and Schmitt and Schmitt [25–27 [25–], a [24]. Consequently, several solutions have been studied to perform the direct C–H carbox- 27],Lewis a Lewis acid [acid28], transition[28], transition metal metal catalysts catalysts [29] and [29] enzymes and enzymes [19,30 [19,30].]. In terms In terms of mechanism, of mech- ylation, by using a base as a reagent, as previously developed by Kolbe and Schmitt [25– anism,those reagentsthose reagents could inﬂuencecould influence the mode the mode of C–H of cleavageC–H cleavage that couldthat could be an be electrophilic an electro- 27], a Lewis acid [28], transition metal catalysts [29] and enzymes [19,30]. In terms of mech- philicaromatic aromatic substitution, substitution, a C–H a deprotonationC–H deprotonation by base by orbase a C–H or a C–H oxidation oxidation and subsequentand subse- anism, those reagents could influence the mode of C–H cleavage that could be an electro- quentCO insertion. CO2 insertion. This This reaction reaction can can take take place place both both in in basic basic or or acidic acidic conditions. conditions. InIn basic philic2 aromatic substitution, a C–H deprotonation by base or a C–H oxidation and subse- media,media, the the use use of of a a strong strong base base deprotonates deprotonates the the C–H C–H group group with with the the most most acidic acidic proton, proton, quent CO2 insertion. This reaction can take place both in basic or acidic conditions. In basic whichwhich is is at at position position 5 5 in in furoic furoic acid acid (FA) (FA) [3 [311],], to form a strong nucleophilic carbon carbon atom, atom, media, the use of a strong base deprotonates the C–H group with the most acidic proton, beingbeing able able to to react react with with weakly weakly electrophilic electrophilic carbon carbon dioxide. dioxide. In acidic In acidic media, media, CO2 COis acti-is which is at position 5 in furoic acid (FA) [31], to form a strong nucleophilic carbon atom,2 vatedactivated via coordination via coordination with with a Lewis a Lewis acid, acid, leading leading to a reaction to a reaction between between the reactant the reactant and being able to react with weakly electrophilic carbon dioxide. In acidic media, CO2 is acti- theand activated the activated CO2 [32]. CO 2This[32 ].reaction This reaction can occur can at occur relatively at relatively low temperatures low temperatures but requires but vated via coordination with a Lewis acid, leading to a reaction between the reactant and highrequires CO2 highpressure, CO2 pressure,and poor andyield poor of the yield target of theproduct target is product reached is due reached to the due different to the theparalleldifferent activated reactions parallel CO2 [32].that reactions couldThis reaction thatoccur. could A can general occur. occur Aschematizationat generalrelatively schematization low of temperatures this process of this isbut illustrated process requires is highinillustrated Figure CO2 2.pressure, in Figure and2. poor yield of the target product is reached due to the different parallel reactions that could occur. A general schematization of this process is illustrated in Figure 2.

FigureFigure 2. 2. PossiblePossible carboxylation carboxylation reaction reaction routes routes from from furo furoicic acid acid(FA) (FA)to 2,5-furandicarboxylic to 2,5-furandicarboxylic acid (FDCA),acid (FDCA), 5-hydroxymethylfurfura 5-hydroxymethylfurfurall (HMF), (HMF), 2,5-dihydroxmethylfu 2,5-dihydroxmethylfuranran (DHMF), (DHMF), 5-hydromethyl-2- 5-hydromethyl-2- Figurefurancarboxylicfurancarboxylic 2. Possible acid acid carboxylation (HFCA), (HFCA), 2,5-diformylfuran reaction routes from (DFF) furo andic 5-formyl-2-furancarboxylic5acid-formyl-2-furancarboxylic (FA) to 2,5-furandicarboxylic acidacid (FFCA). acid (FDCA),(FFCA). 5-hydroxymethylfurfural (HMF), 2,5-dihydroxmethylfuran (DHMF), 5-hydromethyl-2- furancarboxylicIn this context, acid (HFCA), the Henkel 2,5-diformylfuran reaction of (DFF) alkaline and salts5-formyl-2-furancarboxylic of aromatic acids to acid synthesize (FFCA).symmetricalIn this context, diacids the has Henkel been reported reaction in of the alkaline literature salts [33 of,34 aromatic]. This reaction acids to involves synthesize the symmetricalthermal rearrangement diacids has or been disproportionation reported in the lit oferature alkaline [33,34]. salts derived This reaction from aromatic involves acids the thermalto bothIn this therearrangement context, unsubstituted the orHenkel disproportionation and thereaction symmetrical of alkaline of aromaticalkaline salts ofsalts diacids. aromatic derived This acids from process to aromatic synthesize is carried ac- ◦ symmetricalidsout to under both the carbon diacids unsubstituted dioxide has been or and inertreported the atmosphere, symmetrical in the literature at aromatic high pressure,[33,34]. diacids. This between This reaction process 350 involves and is carried 550 theC, thermaloutproducing under rearrangement carbon potassium dioxide terephthalate or ordisproportionation inert atmosphere and benzene ,of at fromalkaline high potassiumpressure, salts derived between benzoate from 350 in aromatic theand presence 550 ac-°C, idsproducingof ato metallic both thepotassium salt unsubstituted (e.g., terephthalate cadmium, and the zinc and symmetrical... benzene)[35–39 from aromatic]. Furthermore, potassium diacids. benzoate the This HCl process acidiﬁcationin the is presence carried of outpotassium under carbon terephthalate dioxide produces or inert atmosphere TA, which is, at used high to pressure, synthesize between PET. 350 and 550 °C, producingAlkaline potassium salts of terephthalate furoic acid disproportionate and benzene from to producepotassium 2,5-FDCA benzoate in in a the similar presence way to that observed in the Henkel reaction for TA synthesis [40–44]. However, formation of

Catalysts 2021, 11, 326 3 of 10

furan is also observed during the reaction. The latter could be hydrogenated to produce 1,4-butanediol [45]. Polycondensation of 1,4-butanediol and 2,5-FDCA can be performed in order to produce poly(1,4-butylene 2,5-furandicarboxylate) (PBF), which is a renewable alternative to PET [46]. For instance, Pan et al. [44] performed a reaction involving potassium furoate as ◦ reactant and ZnCl2 as catalyst, under 38 Bar of CO2, at 250 C, for 3 h. They reported high selectivity of 86% to 2,5-FDCA, with a conversion of 61%. However, purity of 2,5-FDCA was not completely detailed. Regarding the selectivity of the reaction, Thiyagarajan et al. [40,43] recently demonstrated the formation of asymmetrical diacids, as 2,4-FDCA in addition to the 2,5-FDCA. A series of catalytic tests using potassium 2-furoate as reactant in a Kugelrohr glass oven were performed, giving rise to FDCAs’ formation. A yield of up to 91%, using CdI2 as ◦ catalyst, at 260 C, for 5.5 h, under a low flow of N2 [40], was obtained. After esterification of the crude reaction mixture, they demonstrated the presence of both the 2,5-FDCA and the 2,4-FDCA asymmetrical diacids with a 70:30 molar ratio [40]. Several homogeneous or semi-heterogeneous catalysts (as CdI2 and ZnCl2 or Cs2CO3 and K2CO3, respectively) have shown their efficiency in the FDCA synthesis from furoic acid. However, the use of those types of catalysts complicates the purification/separation step of the desired product. The development of heterogeneous catalysts to overcome the homogeneous catalyst problematic could be a solution. The main objective of this work is to study the possibility of using heterogeneous catalysts for the 2,5-FDCA synthesis from furoic acid derivatives. Experimental conditions were set based on those reported in the literature, for the Henkel reaction (devices, operation conditions and reactant/catalyst ratio). Additionally, we discuss herein which operation parameters could be optimized and which kind of heterogeneous catalyst could promote the 2,5-FDCA synthesis.

2. Results 2.1. Validation of the Reaction Setup Regarding the 2,5-FDCA synthesis, one of the main objectives was using a Kugelrohr apparatus, under conditions similar to those reported in the literature [40]. Thiyagarajan et al. [40] performed the production of the 2,5-furan and 2,4-furandicarboxylic acid through Henkel reaction [33,36]. Typically, 10 g of potassium 2-furoate and 22 mol% of CdI2 were mechanically mixed and loaded in a round flask, which was introduced in the Kugelrohr oven. The opti- ◦ mum operating conditions were 260 C during 5.5 h, under a continuous flow of N2 and a slow rotation of the reactor. A similar experiment using 10-times-less reactant and catalyst was performed in our laboratory. A comparison of our results from the NMR analysis with those obtained in the literature is shown in Table1. Furthermore, in order to determine the chemical shift (in ppm) of FA and 2,5-FDCA peaks in DMSO using NMR, pure compounds were analyzed separately (Supplementary Materials Figures S1 and S2, respectively).

Table 1. Validation of the experimental setup for the Henkel reaction, using the Kugelrohr apparatus.

Experiment XK2F (%) FDCAs Formation (%) S2,5-FDCA (%) S2,4-FDCA (%) Literature [40] 92 91 70 30 This work 73 69 69 31 −1 ◦ Conditions: 530 mg of CdI2, 1 g of K2F, FN2 = 45 mL min , 20 rpm, T = 260 C, t = 5.5 h.

Results calculated from NMR analysis (Supplementary Materials Figure S3) showed a K2F conversion and a 2,5-FDCA formation slightly lower than that obtained by Thiyagarajan et al. [40]. However, the individual selectivity of furandicarboxylic acids remains of the same order. Regarding the results, it was concluded that the Kugelrohr glass oven is suitable for the Henkel reaction tests. Catalysts 2021, 11, 326 4 of 10

2.2. Dual Catalytic System (Ag/SiO2 + CdI2) The Henkel reaction to produce 2,5-FDCA from K2F has shown good results using ◦ CdI2 as catalyst [40]. However, the reaction temperature must reach at least 260 C to achieve good conversions. At this temperature, CdI2 starts to decompose (melting point is 387 ◦C), thus leading to a better interaction between the solid K2F and the semi-melted catalyst. In order to decrease the working temperature, one of the proposed solutions was to use a heterogeneous catalyst in addition to the CdI2 catalyst, in CO2 atmosphere. The temperature was decreased from 260 to 200 ◦C, to observe the evolution of the results compared to those in the literature [40]. Results are shown in Table2.

Table 2. Inﬂuence of temperature for the 2,5-FDCA synthesis using Ag/SiO2 and CdI2 mixture on the K2F conversion and space–time yields (STY).

Temperature Conversion STY STY Entry Catalyst 2,5-FDCA DFF (◦C) (%) (µmol kg−1 h−1) (µmol kg−1 h−1)

1 CdI2 200 0 - - 2 Ag/SiO2/CdI2 200 51 264 951 3 Ag/SiO2/CdI2 230 74 145 - 4 Ag/SiO2/CdI2 260 69 188 - 5 Ag/SiO2 200 20 1203 - −1 ◦ Conditions: 17 mg of CdI2, 35 mg of K2F, 50 mg of Ag/SiO2,FCO2 = 45 mL min , 20 rpm, T = 200–260 C, t = 20 h.

Using a dual catalytic system seems to open the possibility to significantly decrease the reaction temperature, since the K2F conversion does not show a significant decrease (Table2 , Entries 2 and 4). However, 2,5-FDCA’s yield is more affected by using lower temperature. Indeed, at 200 ◦C, the formation of the 2,5-diformyl furan (DFF) was observed. It was also the major product at this temperature. However, it confirms the beneficial effect ◦ of the Ag/SiO2, since no activity was observed at 200 C for CdI2 alone (Table2, Entry 1). In addition, Ag/SiO2 alone already produces 2,5-FDCA, even if the K2F conversion decreases significantly (Table2, Entry 5).

2.3. Effect of the Support Taking into account good preliminary results obtained with the Ag/SiO (Table2), Catalysts 2021, 11, x FOR PEER REVIEW 2 5 of 11 the screening of the different supports was performed. Silver and gold catalysts supported on different supports were tested, and the results are shown in the Figure3.

1500 40 ) 1 − 1200 h

1 30 − 900 20 600 X K2F (%) K2F X 10 300

STY FDCA (µmol kg STY FDCA (µmol 0 0

O 2 2 2 2 g gO eO eO M C C Au/SiO Ag/SiO Au/M Ag/ Au/ Ag/ FigureFigure 3. 3.Effect Effect of of the the support support on goldon gold and silverand silver catalysts catalysts in the K2F in the carboxylation: K2F carboxylation: K2F conversion K2F conver- −1 −1 (sion•) and (●) the and STY the of STY 2,5-FDCA of 2,5-FDCA () (Conditions: (■). (Conditions: Substrate/M Substrate/M = 9, FCO =2 9,= F 45CO2 mL = 45 min mL ,min 20 rpm,, 20 rpm, T = T200 = 200 °C,◦ C,t = t20 = 20h.) h).

Regarding K2F conversion, the use of Au instead of Ag increased the activity, what- ever the support. However, a lower 2,5-FDCA yield was obtained by using Au catalysts. Regarding the support, the use of MgO with Au or Ag leads to a low 2,5-FDCA production. On the contrary, Ag/CeO2 and Ag/SiO2 catalysts have shown similar activity (~ 20% of K2F conversion) and promote 2,5-FDCA production up to 524 and 1203 µmol kg−1 h−1, respectively. In order to explain these results, it has to be considered that CeO2 support has basic and redox properties, while MgO presents high basicity [47]. The redox properties of the CeO2 could be the reason of an enhanced performance. Indeed, the oxygen vacancies of CeO2 increase the adsorption capacity of CO2 [47,48], which could promote the carboxylation reaction. However, the use of SiO2 has shown a much better yield to 2,5- FDCA than CeO2. That could be explained by the presence of acid sites in the support [49].

2.4. Effect of the Metal

Since the SiO2 support has shown the best performances towards 2,5-FDCA synthesis, it has been selected for the screening of the metal phase in the heterogeneous catalyst. A series of M/SiO2 catalysts were tested, in similar conditions, for comparison. K2F conversion and 2,5-FDCA space–time yield are shown in Figure 4.

1500 50 ) 1 − 1200 40 h 1 − 900 30

600 20 X K2F (%)

300 10

STY FDCA (µmol kg STY FDCA (µmol 0 0

2 2 2 2 2 2 2 iO iO iO /SiO S /SiO /SiO /S /SiO /S Ni/ n Co Z Ag Cd Cs Au

Figure 4. Effect of the metal, using SiO2 supported catalysts on the conversion (●) of K2F and the STY of 2,5-FDCA (■). (Conditions: Substrate/M = 9, FCO2 = 45 mL min−1, 20 rpm, T = 200 °C, t = 20 h).

Catalysts 2021, 11, x FOR PEER REVIEW 5 of 11

1500 40 ) 1 − 1200 h

1 30 − 900 20 600 X K2F (%) K2F X 10 300

STY FDCA (µmol kg STY FDCA (µmol 0 0

O 2 2 2 2 g gO eO eO M C C Au/SiO Ag/SiO Au/M Ag/ Au/ Ag/ Figure 3. Effect of the support on gold and silver catalysts in the K2F carboxylation: K2F conversion (●) and the STY of 2,5-FDCA (■). (Conditions: Substrate/M = 9, FCO2 = 45 mL min−1, 20 rpm, T = 200 °C, t = 20 h.) Catalysts 2021, 11, 326 5 of 10 Regarding K2F conversion, the use of Au instead of Ag increased the activity, what- ever the support. However, a lower 2,5-FDCA yield was obtained by using Au catalysts. RegardingRegarding the K2Fsupport, conversion, the use the of use MgO of Au with instead Au or of Ag increasedleads to a the low activity, 2,5-FDCA what- produc- ever the support. However, a lower 2,5-FDCA yield was obtained by using Au catalysts. tion. On the contrary, Ag/CeO2 and Ag/SiO2 catalysts have shown similar activity (~ 20% Regarding the support, the use of MgO with Au or Ag leads to a low 2,5-FDCA production. of K2F conversion) and promote 2,5-FDCA production up to 524 and 1203 µmol kg−1 h−1, On the contrary, Ag/CeO2 and Ag/SiO2 catalysts have shown similar activity (~20% of K2Frespectively. conversion) In andorder promote to explain 2,5-FDCA these production results, it uphas to to 524 beand considered1203 µmol that kg− CeO1 h−12, support has basic and redox properties, while MgO presents high basicity [47]. The redox proper- respectively. In order to explain these results, it has to be considered that CeO2 support has basicties of and the redox CeO properties,2 could be whilethe reason MgO presents of an enhanced high basicity performance. [47]. The redox Indeed, properties the oxygen of va- thecancies CeO 2ofcould CeO be2 increase the reason the of adsorption an enhanced capacity performance. of CO Indeed,2 [47,48], the which oxygen could vacancies promote the ofcarboxylation CeO2 increase reaction. the adsorption However, capacity the of use CO 2of[47 SiO,48],2 whichhas shown could promotea much thebetter carboxy- yield to 2,5- lationFDCA reaction. than CeO However,2. That could the use be of explained SiO2 has shown by the a muchpresen betterce of yield acid tosites 2,5-FDCA in the support than [49]. CeO2. That could be explained by the presence of acid sites in the support [49].

2.4. EffectEffect of of the the Metal Metal Since the SiO2 support has shown the best performances towards 2,5-FDCA synthe- Since the SiO2 support has shown the best performances towards 2,5-FDCA synthesis, itsis, has it has been been selected selected for the for screening the screening of the of metal the metal phase phase in the heterogeneousin the heterogeneous catalyst. catalyst. A seriesseries ofof M/SiOM/SiO22 catalystscatalystswere were tested, tested, in in similar similar conditions, conditions, for for comparison. comparison. K2F K2F con- conversionversion and and 2,5-FDCA 2,5-FDCA space–time space–time yield areare shown shown in in Figure Figure4. 4.

1500 50 ) 1 − 1200 40 h 1 − 900 30

600 20 X K2F (%)

300 10

STY FDCA (µmol kg STY FDCA (µmol 0 0

2 2 2 2 2 2 2 iO iO iO /SiO S /SiO /SiO /S /SiO /S Ni/ n Co Z Ag Cd Cs Au

Figure 4. 4.Effect Effect of of the the metal, metal, using using SiO2 SiOsupported2 supported catalysts catalysts on the conversionon the conversion (•) of K2F ( and●) of the K2F STY and the −1 −1 ◦ ofSTY 2,5-FDCA of 2,5-FDCA () (Conditions: (■). (Conditions: Substrate/M Substrate/M = 9, FCO2 == 9, 45 F mLCO2 min= 45 mL, 20 min rpm,, T 20 = 200rpm,C, T t= = 200 20 h).°C, t = 20 h). High 2,5-FDCA production was observed only for Ag/SiO2 catalyst with a conversion of 20% (Figure4). Cs and Co supported on silica showed the lowest conversion values, between 17 and 25%. On the other hand, Zn and Cd supported on silica presented the highest conversion values, which are 47 and 35% respectively. Both metals have already shown promising results in the Henkel reaction, using CdI2 and ZnCl2. Regarding the metal composition, only the catalyst containing Ag has shown interesting results in 2,5-FDCA synthesis, contrary to the other metals tested. An XRD diffractogram of the Ag/SiO2 catalyst is shown in Supplementary Materials Figure S4. ◦ Diffraction peaks located at 34, 49 and 61 2θ could correspond to Ag2O (JCDS ICDD ◦ 00-042-9874) or Ag2CO3 (JCDS ICDD 04-017-5597), while the three other peaks at 38.1 , 44.2◦ and 64.4◦ 2θ are representative of the (111), (200) and (220) planes of metallic Ag (JCDS ICDD 00-001-1164), respectively. The mean crystallite sizes of Ag metallic and Ag2O, calculated using Scherrer’s equation, were of 25.3 and 9.5 nm, respectively. One of the main differences between silver and the others metals comes from its particular electronic conﬁguration ([Kr] 4d10 5s1 from group 11), typical from the so-called “coinage metals” group. Furthermore, it has been reported in the literature that Ag(I) salts promote the carboxylation of terminal alkynes [50–52]. The d10 electronic conﬁguration of silver favors the activation of alkynes through its interaction with the C–C π-bond of alkyne. Catalysts 2021, 11, 326 6 of 10

Moreover, Lui et al. [53] have demonstrated that the use of heterogeneous Ag@MIL-101 catalysts promotes the capture and conversion of CO2, overcoming the need of strong base or aggressive organometallic reagents to activate the hydrogen of the terminal alkyne. It was also shown that the adsorption of gaseous CO2 on the Ag surface occurs differently compared to other metals such as Cu or Au. In case of oxidized Ag atoms, surface O atoms interact with gaseous CO2 and form chemisorbed on the surface of the metal carbonic − acid-like species. In these carbonic acid-like species, two oxygen atoms from CO3 are bonded to adjacent Ag bridging sites. The third oxygen atom forms a C double bond δ− (C = O) perpendicular to the surface. This carbonic O = CO2 surface specie has a negative charge localized on the two oxygen atoms binding to the Ag surface [54]. The presence of Ag2O could explain the 2,5-FDCA formation, which is produced only by using Ag catalysts. As previously reported, the use of Ag(I) promotes alkynes carboxylation. Consequently, in our case 2,5-FDCA formation could come from the interaction of Ag+ and the reactant. Moreover, this hypothesis could explain the low 2,5-FDCA formation on Au catalysts, which only present metallic gold nanoparticles [55,56]. This parameter would need some deeper investigation to understand the role of silver in the reaction. In conclusion, an effect of the support was observed from using supports with different acidity and redox properties. Supports with acidic sites and redox properties promote the production of 2,5-FDCA, while basic supports, like MgO, were demonstrated to have the lower catalytic performances. On the other hand, the screening of different metals supported on SiO2 has shown that only Ag is leading to the formation of 2,5-FDCA, but with less conversion than Au, Zn, Cd and Ni. Therefore, for the optimization of the reaction conditions, the Ag/SiO2 catalyst was used.

2.5. Effect of the Substrate/Metal Molar Ratio Catalysts 2021, 11, x FOR PEER REVIEW 7 of 11 In this section, the substrate/Ag molar ratio from 1 to 33 was studied. The catalytic results (K2F conversion and space–time yield (STY) of 2,5-FDCA) are presented in Figure5.

1500 40 ) 1 − 1200 h

1 30 − 900 20 600 X K2F (%) X 10 300

STY FDCA (µmol kg (µmol FDCA STY 0 0 33 17 9 8 5 1 Substrate/Ag

Figure 5.5. EffectEffect ofof substrate/Ag substrate/Ag molar molar ratio ratio on on the the conversion conversion of K2Fof K2F (•) ( and●) and the the STY STY of 2,5-FDCA of 2,5-FDCA −1−1 ◦ (■).) (Conditions:(Conditions: Substrate/Ag Substrate/Ag = = 1–33, 1–33, F COFCO22 = = 45 45 mL mL min min ,, 2020 rpm, rpm, T T = = 200 200 C,°C, t =t = 20 20 h). h.)

As expected,expected, the the K2F K2F conversion conversion increases increases when when the the substrate/Ag substrate/Ag molar molar ratio ratio de- de- creases. These results could be directly linked to a limitation of the available active sites on creases. These results could be directly linked to a limitation of the available−1 active−1 sites the Ag/SiO2 catalyst. Regarding the 2,5-FDCA STY, a maximum of 1260 µmol kg h is on the Ag/SiO2 catalyst. Regarding the 2,5-FDCA STY, a maximum of 1260 µmol kg−1 h−1 obtained by using a substrate/Ag molar ratio equal to 26. The molar ratio of 9 seems to is obtained by using a substrate/Ag molar ratio equal to 26. The molar ratio of 9 seems to be an optimum value due to a 2,5-FDCA STY of 1203 µmol kg−1 h−1 near to the highest −1 −1 bevalue, an andoptimum to a K2F value conversion due to ofa 2,5-FDCA 20% instead STY of 9%of (obtained1203 µmol for kg the h molarnear ratio to the of 33).highest value,Surprisingly, and to the a K2F 1, 5 conversion and 17 molar of ratios 20% instead have given of 9% a very (obtained low formation for the molar of 2,5-FDCA, ratio of 33). whichSurprisingly, could come the from1, 5 and a lack 17 of molar physical ratios interaction have given of the a mixture,very low together formation with of a kinetic2,5-FDCA, whichlimitation could for thecome formation from a oflack 2,5-FDCA. of physical interaction of the mixture, together with a kinetic limitation for the formation of 2,5-FDCA.

2.6. Effect of Reaction Temperature The reaction temperature has been studied from 170 to 300 °C. The conversion and space–time yield are shown in Figure 6.

1500 70 )

1 60 − 1200 h 1

− 50

900 40

600 30

20 (%) K2F X 300 10

STYFDCA (µmol kg 0 0 170 200 230 260 300

Temperature (°C)

Figure 6. Influence of the reaction temperature on the catalytic performance of Ag/SiO2 with the

conversion of K2F (●) and the STY of 2,5-FDCA (■). (Conditions: Substrate/Ag = 9, FCO2 = 45 mL min−1, 20 rpm, T = 170–300 °C, t = 20 h.)

At 170 °C, a very low conversion of 6% was obtained. An increase of the temperature enhances conversion, with an optimum 2,5-FDCA space–time yield of 1203 µmol kg−1 h−1 at 200 °C. Furthermore, between 230 and 300 °C, an increase in K2F conversion to 2,5- FDCA reaching 65% was observed. However, only 715 µmol kg−1 h−1 of 2,5-FDCA yield at 300 °C was obtained.

Catalysts 2021, 11, x FOR PEER REVIEW 7 of 11

1500 40 ) 1 − 1200 h

1 30 − 900 20 600 X K2F (%) X 10 300

STY FDCA (µmol kg (µmol FDCA STY 0 0 33 17 9 8 5 1 Substrate/Ag

Figure 5. Effect of substrate/Ag molar ratio on the conversion of K2F (●) and the STY of 2,5-FDCA (■). (Conditions: Substrate/Ag = 1–33, FCO2 = 45 mL min−1, 20 rpm, T = 200 °C, t = 20 h.)

As expected, the K2F conversion increases when the substrate/Ag molar ratio decreases. These results could be directly linked to a limitation of the available active sites on the Ag/SiO2 catalyst. Regarding the 2,5-FDCA STY, a maximum of 1260 µmol kg−1 h−1 is obtained by using a substrate/Ag molar ratio equal to 26. The molar ratio of 9 seems to be an optimum value due to a 2,5-FDCA STY of 1203 µmol kg−1 h−1 near to the highest value, and to a K2F conversion of 20% instead of 9% (obtained for the molar ratio of 33). Surprisingly, the 1, 5 and 17 molar ratios have given a very low formation of 2,5-FDCA, Catalysts 2021, 11, 326 which could come from a lack of physical interaction of the mixture, together7 of 10with a kinetic limitation for the formation of 2,5-FDCA.

2.6. EffectEffect of of Reaction Reaction Temperature Temperature TheThe reaction reaction temperature temperature has has been been studied studied from from 170 to 170 300 to◦C. 300 The °C. conversion The conversion and and space–time yield yield are are shown shown in Figurein Figure6. 6.

1500 70 )

1 60 − 1200 h 1

− 50

900 40

600 30

20 (%) K2F X 300 10

STYFDCA (µmol kg 0 0 170 200 230 260 300

Temperature (°C)

Figure 6. 6.Inﬂuence Influence of of the the reaction reaction temperature temperature on the on catalytic the catalytic performance performance of Ag/SiO of2 with Ag/SiO the con-2 with the − conversion of K2F (●) and the STY of 2,5-FDCA (■). (Conditions: Substrate/Ag = 9, FCO2 = 145 mL version of K2F (•) and the STY of 2,5-FDCA () (Conditions: Substrate/Ag = 9, FCO2 = 45 mL min , 20min rpm,−1, 20 T =rpm, 170–300 T = 170–300◦C, t = 20 °C, h). t = 20 h.)

◦ AtAt 170 170 C,°C, a a very very low low conversion conversion of 6% of was6% obtained.was obtained. An increase An increase of the temperature of the temperature µ −1 −1 enhances conversion, conversion, with with an optimuman optimum 2,5-FDCA 2,5-FDCA space–time space–time yield of 1yield203 ofmol 1203 kg µmolh kg−1 h−1 at 200 ◦C. Furthermore, between 230 and 300 ◦C, an increase in K2F conversion to 2,5-FDCA at 200 °C. Furthermore, between 230 and 300 °C, an increase in K2F conversion to 2,5- reaching 65% was observed. However, only 715 µmol kg−1 h−1 of 2,5-FDCA yield at 300 ◦C −1 −1 wasFDCA obtained. reaching 65% was observed. However, only 715 µmol kg h of 2,5-FDCA yield at 300 °C was obtained. 3. Materials and Methods 3.1. Catalysts Preparation A series of Au and Ag catalysts with a nominal metal content of 7 wt.% were prepared by wet impregnation, using water as solvent and using commercial SiO2 (CARiACT Q-10), CeO2 (Sigma-Aldrich, Saint Louis, MO, USA) and MgO (Sigma-Aldrich, Saint Louis, MO, −1 USA) as support. Typically, for 1 g of catalyst, 10 mL of a 66 mmol L of AgNO3 solution (99%, Sigma-Aldrich, Saint Louis, MO, USA) was added to 0.93 g of the support. The mixture was kept under stirring (150 rpm), overnight, and the solvent was evaporated at 90 ◦C, using a vacuum, prior to a drying step at 80 ◦C, overnight. The obtained solids were afterwards calcined in air atmosphere for 4 h, at 300 ◦C. M/SiO2 catalyst series with M = Zn, Cs, Cd, Ni and Co were prepared by using the same method, by adjusting the concentration of the precursor solution in order to obtain a 7 wt.% metal content.

3.2. Catalytic Test Potassium 2-furoate (K2F) was prepared by using furoic acid (97%, Sigma-Aldrich, Saint Louis, MO, USA) and KOH (85%, Sigma-Aldrich, Saint Louis, MO, USA) in a 1:1 molar ratio. 2,5-FDCA synthesis was performed in a Glass Oven B-585 Kugelrohr (Büchi), under CO2 atmosphere. Typically, 1 mmol of K2F was mechanically mixed with 100 mg of Ag/SiO2 and introduced into a round flask and placed inside the oven. Then, the ◦ solid mixture was slowly rotated at 20 rpm, at 200 C, under a continuous flow of CO2 (45 mL min−1), during 20 h. Afterwards, the setup was cooled down for 1 h. The obtained black solid was dispersed in H2O or MeOH, stirred for 1 h and filtered by using PTFE (2 µm), giving a pale-yellow filtrate and the remaining catalyst. Catalysts 2021, 11, 326 8 of 10

To analyze the reactions reagents and products, a gas chromatograph (GC, Agilent 7890B) apparatus equipped with a CP-Wax 52 CB GC column or a High-Performance Liquid Chromatography (HPLC, Waters 2410 RJ) apparatus equipped with a Shodex −1 SUGAR SH-1011 column and UV detector, using 5 mM of H2SO4 (0.6 mL min ) as a mobile phase, was used. Conversion and STY were calculated by using the following formulas, Formulas (1) and (2), respectively.

XK2F = (n0K2F − nK2F)/n0K2F × 100, (1)

where XK2F is the potassium furoate conversion (%), and n0K2F and nK2F are the initial and the non-reacted number of moles of potassium furoate, respectively (mol).

STYi = ni/(mcatalyst × tr), (2)

−1 −1 where STYi is the space–time yield of a product i (µmol kg h ), ni is the number of moles of product i (µmol), mcatalyst is the mass of catalyst (kg) and tr is the reaction time (h).

4. Conclusions

A dual catalytic system containing a heterogeneous catalyst and CdI2, a semi-homogeneous catalyst, was tested. Preliminary results conﬁrmed the possibility to decrease the reaction temperature to 230 ◦C, obtaining an acceptable conversion (74%). However, a drastic decrease of the 2,5-FDCA STY was observed from 8489 to 154 µmol kg−1 h−1, which could be explained by a high adsorption of the products on the heterogeneous catalyst. It is worth noting that an increase in the temperature of the reaction disfavored the adsorption and favored the catalytic conversion. Several experiments have been performed to screen a support and a metal which could promote 2,5-FDCA production. SiO2 support has shown the greatest promotion of 2,5-FDCA synthesis from K2F, contrary to CeO2 and MgO. In addition, Ag/SiO2 leads to the highest yields in 2,5-FDCA formation, while other monometallic (Ni, Co, Zn, Cd ... ) catalysts showed much lower productivities.

Supplementary Materials: The following are available online, at https://www.mdpi.com/2073-434 4/11/3/326/s1. Figure S1: Representative 1H-NMR of furoic acid (FA). Figure S2: Representative 1H-NMR of isolated FDCA. Figure S3: Representative 1H-NMR of crude products obtained from the carboxylation of K2F to FDCA. Figure S4: XRD diffractogram of Ag/SiO2. Author Contributions: Conceptualization, F.D., Y.S. and R.W.; methodology, F.D.; formal analysis, F.D. and J.T.-R.; investigation, F.D. and Y.S.; resources, R.W.; data curation, F.D., J.T.-R., I.I.J. and R.W. writing—original draft preparation, F.D., and R.W.; writing—review and editing, F.D.; I.I.J., S.P. and R.W.; visualization, I.I.J. and S.P.; supervision, R.W.; project administration, R.W.; funding acquisition, R.W. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by REGION HAUTS-de-FRANCE, grant number 3859523 FDCA STARTAIRR project and I-SITE ULNE grant V-Start’AIRR-18-001-Wojcieszak This study was supported by the French government through the Programme Investissement d’Avenir (I-SITE ULNE/ANR-16- IDEX-0004 ULNE) managed by the Agence Nationale de la Recherche. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: The REALCAT platform is benefiting from a state subsidy administrated by the French National Research Agency (ANR), within the frame of the “Future Investments” program (PIA), with the contractual reference “ANR-11-EQPX-0037”. The European Union, through the ERDF funding administered by the Hauts-de-France Region, co-financed the platform. Centrale Lille, the CNRS and Lille University, as well as the Centrale Initiative Foundation, are thanked for their financial contribution to the acquisition and implementation of the equipment of the REALCAT platform. The Métropole Européen de Lille (MEL) for the “CatBioInnov” project is also acknowledged. Conflicts of Interest: The authors declare no conflict of interest. Catalysts 2021, 11, 326 9 of 10

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